• Power is defined as

 

The rate of energy transfer or the amount of energy transferred per second

 

• The power of a device depends on:
  • The?voltage?(potential difference) of the device
  • The?current?of the device

   

• The power of an electrical component (or appliance) is given by the equation:

 

• The unit of power is the?Watt?(W), which is the same as a joule per second (J/s)

 

 

 

• This equation can be rearranged with the help of a formula triangle:

Power, current, voltage formula triangle

 

Worked Example

 

Step 1: List the known quantities

• • Power,?P?= 48 W
  • Current,?I?= 4 A

   

 

Step 2: Write down the relevant equation

 

P = IV

 

Step 3: Rearrange for potential difference,?V

Step 4: Substitute in the values

 

 

 

Exam Tip

 

• A fuse is a safety device designed to?cut off the flow of electricity to an appliance if the current becomes too large?(due to a fault or a surge)

The circuit symbol for a fuse - take care not to confuse this with a resistor

• Fuses usually consist of a glass cylinder which contains a thin metal wire
• If the current in the wire becomes too?large:
  • The wire heats up and?melts
  • This causes the wire to?break, breaking the circuit and stopping the current

   

• This makes sure that more current doesn't keep flowing through the circuit and causing more damage to the equipment, or, causing a fire

 

 

 

• Fuses come in a?variety?of sizes, typically 3 A, 5 A and 13 A
  • In order to select the right fuse for the job, the?current?through an appliance needs to be known

   

• If the?power?of the appliance is known (along with?mains?voltage), the current can be calculated using the equation:

 

• Where:
  • I?= current in amps (A)
  • P?= power in watts (W)
  • V?= voltage in volts (V)

   

• The fuse should always have a current rating that is?higher?than the current needed by the appliance, without being too high
  • Because of this, the rule of thumb is to?always?choose the?next size up

   

• If the fuse current rating is low, it will break the circuit even when an acceptable current is flowing through
• If the fuse current rating is too high, it will not be breaking the circuit in enough time before damage occurs

 

Worked Example